The physics of light
spin-1 dark bosons
has witnessed a growing amount of interest in the recent years,
from both a theoretical and phenomenological standpoint.
A standard benchmark
is that of a
secluded \({\rm U}(1)\)  gauge boson, kinetically mixed with the photon [1]} and hence
universally coupled to the Standard Model (SM) sector via the electromagnetic current.
This framework, although elegant and predictive, can be
too restrictive for phenomenological applications
and hence more general forms of the light vector boson interactions with the SM fields can be envisaged.
Going beyond the kinetic mixing framework, a theoretically motivated
option is provided by the gauging of the accidental global symmetries of the SM,
that is baryon number \({\rm U}(1)_B\)  and family lepton number \({\rm U}(1)_{L_i}\)  (with \(i = e,\mu ,\tau \) )
in the limit of massless neutrinos.
Within the SM field content, only the \(L_i-L_j\)  combinations turn out to be anomaly free [2]}, [3]}, [4]}. Hence,
in order to consistently gauge a general linear combination
\(X = \alpha _B B + \sum _{i=e,\, \mu ,\, \tau } \alpha _i L_i \, ,\) 
